The purpose of this trial is to evaluate the role of calcium supplementation in the prevention of preeclampsia and pre-term delivery. The protocol will involve a randomized double-blind placebo controlled trial in 900 nulliparous pregnant women. Two grams of calcium carbonate or placebo will be given per day beginning by week 21 and patients will be followed prospectively for endpoints such as pregnancy aggravated hypertension, pregnancy aggravated proteinuria, preeclampsia and urolithiasis.